Lepidopteran stalk borers are the main pests that severely damage sugarcane in many sugarcane producing countries. Larvae bore either into the shoots or stalks of sugarcane, severely reducing both yield and sugar content (Sallam et al., 2010; Goebel et al., 2011; McGuire et al., 2012; Sattar et al., 2016). Tetramoera schistaceana (Snellen) and Chilo sacchariphagus (Bojer) are widely distributed in planting areas and affecting both the yield and quality of sugarcane in China (Huang and Li, 2011; Leul and Thangavel, 2013). This has become more important in recent years, because the consistently warmer winters and exchange of introduced sugarcane varieties between areas has resulted in a change in the species, occurrence and extent of damage from T. schistaceana and C. sacchariphagus (Yao et al., 2006; An and Guan, 2009; Xiong et al., 2010; Xie et al., 2012; Li et al., 2013). The main changes include infestation from combination of borers, a year-on-year increase in population density, a sharp increase in percent dead-heart and percent stalks damaged in the middle and late stage, and a year-on-year increase in loss of cane and sugar yield which causes considerable economic loss to the main sugarcane planting areas (Yao et al., 2006; An and Guan, 2009; Xiong et al., 2010; Xie et al., 2012; Li et al., 2013). Determining the cane yield and sugar losses caused by T. schistaceana and C. sacchariphagus are essential to for formulate the relevant control strategy For example, the entire sugarcane growing period could be affected by damage from these borer species; the borers cause dead hearts in the seedling stage, and the number of seedlings and cane stalks may be reduced (Feng, 1999; Zhang et al., 2008; Li et al., 2014). In the middle and later growing stages, T. schistaceana and C. sacchariphagus attack sugarcane stems and destroy internal tissue of the stalk; this affects the joint growth and leads to the decrease in cane and sugar yield (Feng, 1999; Zhang et al., 2008; Li et al., 2014). The percent dead-heart in the seedling stage can reach as much as 30% in severely damaged fields, and the percent of stalks damaged can reach above 40% (Pan et al., 2009; Lu et al., 2011; Huang et al., 2014). Many studies have been published about the estimation of sugarcane yield losses due to borers in many counties (Rajabalee et al., 1990; Goebel and Way, 2003; Reay-Jones et al., 2005; White et al., 2008; Rossato et al., 2013; Goebel et al., 2014). However, studies assessing loss of sugarcane yield due to the occurrence of mixed populations of T. schistaceana and C. sacchariphagus under natural field conditions have not been reported until now in China, so there is a considerable lack of data on this aspect. In this study, the losses of sugarcane yield and sugar yield caused by the occurrence of T. schistaceana and C. sacchariphagus and the impact of damage of sugarcane borers was assessed under the natural field conditions.

Materials and Methods

Field experimental design

The field experiments of newly planted and ratoon cane were carried out in February 2014 to January 2015 and February 2015 to January 2016 respectively using the main cultivars, ROC22, ROC25, Yuetang 83-88, Yuetang 93-159, Yuetang 00-236 and Yingyu 91-59. Experiments were located in Lincang, Yunnan Province in China. Experimental field is located at 23°47’ N and 99° 36’ E, 1157 meter above sea level. Treated and untreated areas within the same field were examined in three replicates a total of six experimental plots. Each plot was 167-333 m2 (depending on the size of the field). Plots were arranged in a randomized block design. Soil conditions, fertility, water and fertilizer management, as well as growth of seedlings were comparable between the treated and untreated plots.

In treated plots, 90 kg/ha 3.6% Bisultap GR was added in February–March when planting or loosening the soil of ratoon cane took place, and May–June when the soil was ‘hilled up’, respectively. 3.6% Bisultap GR was mixed with the 1200 kg/ha NPK 20-10-10 fertilizerand spread evenly across sugarcane ditches, sugarcane stump or base of stalks and covered with soil or plastic film. In untreated plots, fertilizer only was applied. All other cultivation management measures were conducted according to the local conventional production methods and were the same of both areas.

Borer damage investigation

The percent dead-hearts was measured in both treated and untreated in June. A five-point sampling method was employed in each plot: at each of five points, 100 plants were selected sequentially to investigate, a total of 500 plants.

The percent dead-hearts was calculated as follows:

Percent dead-hearts (%) = Number of dead heart /

Total number of surveyed seedlings ×100

At the sugarcane mature stage, the percent stalk damaged and percent internodes bored in the treated and untreated area were investigated in December. A five-point sampling method was used in each plot: at each of five points 20 stalks were selected sequentially, a total of 100 stalks. The leaf sheath of each stalk was stripped and the total number of damaged stalks was recorded. To assess the percent internodes bored, 20 stalks were randomly selected from the 100 stalks. The total number of internodes on each stalk and the number of internodes damaged were recorded. The percent stalk damaged and the percent internodes bored were calculated as follows:

Number of borer-damaged internode / Total number of surveyed internodes ×100.

At the sugarcane mature stage, the effective stem (more than 1 meter in length) number in the treated and untreated area was assessed in December. Three point sampling method was used in each plot: at each of three points, the average row spacing of five rows of sugarcane were measured and 10 m of row length of each row was chosen to evaluate the total number of effective stems. The number of effective stems per hectare was calculated as follows:

In January when the crops were harvested, the cane yield from treated and untreated areas was assessed. The cane biomass was weighted form 66 m2 central areas in each plot after cutting and the relative yield loss percent was calculated as follows:

When the crops were harvested in January, the sucrose content of treated and untreated area was determined. 10 sugarcane stalks were randomly selected in each plot. Using the two times polarimetric analysis method established by the National Sugar Industry Standardization and Quality Detection Center was used to determine quality indexes including juice yield (%), sucrose content (%), fiber content (%), juice brix (°BX) , gravity purity (%) and reducing sugar content (%). A fully automatic sugar analysis system, Rudolph, Autopol 880+J257 (United States), was used. The loss of each index was calculated as follows:

The differences between treated area and untreated area were analyzed with one-way ANOVA followed by Tukey’s HSD test (SAS, 2001). The arcsine transformation was performed on percentages prior to analysis. We set the level of significance to P < 0.05 for all statistical tests.

Results

The influence of T. schistaceana and C. sacchariphagus-damage on yield

T. schistaceana and C. sacchariphagus have caused severe damage in cane-growing regions of China that were showed from Figure 1 and Table I. The occurrence and degree of damage by sugarcane borer in different sugarcane planting areas varied. In the newly planted field, the average percent dead-heart, the average percent stalk damaged and the average percent internodes bored in the untreated area were increased by 12.31%, 26.13%, 6.85%, respectively significantly higher than those in the treated areas, and the average effective stems number and average yield of sugarcane in the untreated areas were reduced by 10895 stalk/ ha and 15.59 T/ha, respectively significantly less than those in the treated areas. In the ratoon field, the average percent dead-heart, the average percent stalk damaged and the average percent internodes bored in the untreated area were increased by 37.49%, 67.80%, 14.53%, respectively significantly higher than those in the treated areas, and the average effective stems number and average yield of sugarcane in the untreated areas were reduced by 19967 stalk/ ha and 38.59 T/ha, respectively significantly less than those in the treated areas.

The influence of damage by T. schistaceana and C. sacchariphagus on sugarcane quality

The results of the sugarcane quality analyzes are listed in Table II. In the newly planted field, the average sugarcane juice yield, average sucrose content, average juice brix and average juice gravity purity of damaged sugarcane in the untreated areas were reduced by 2.73%, 1.04%, 1.43 and 1.40%, respectively significantly less than those in the treated areas, but the fibre and reducing sugar content of damaged sugarcane in the untreated area were increased by 0.48% and 0.29% respectively significantly higher than those in the treated areas. In the ratoon field, the average sugarcane juice yield, average sucrose content, average juice brix and average juice gravity purity of damaged sugarcane in the untreated areas were reduced by 2.58%, 2.62%, 2.57 and 5.52%, respectively significantly less than those in the treated areas, but the fibre and reducing sugar content of damaged sugarcane in the untreated area were increased by 0.69% and 0.73%, respectively significantly higher than those in the treated areas.

Thus, the quality of sugarcane was affected by the damage by T. schistaceana and C. sacchariphagus in varying degrees, which resulted in a reduction of sugar yield.

Discussion

T. schistaceana and C. sacchariphagusare major stalk borers which are widely distributed in sugarcane planting fields in China, causing severe damage to the plant and easily transmitted by vegetative propagation of sugarcane (Huang and Li, 2011; Leul and Thangavel, 2013). Climate change could alter patterns of disturbance from pest insects through direct effects on their development and survival, adaptation capability, availability of host plants and physiological changes in host defenses, and indirect effects from changes in the abundance of natural enemies, mutualists, and competitors (Bergant et al., 2005). In recent years, the global climate warming and the exchange of sugarcane cultivars between different areas have led to changes in the species, occurrence and extent of damage caused by T. schistaceana and C. sacchariphagus in main cane-growing areas, such as, Guangxi, Yunnan, Guangdong, and Hainan in China (Yao et al., 2006; Xiong et al., 2010; Xie et al., 2012; Li et al., 2014). The infestation of sugarcane borers has become increasingly severe causing great economic loss. It is therefore important to correctly understand the effect of T. schistaceana and C. sacchariphagus on sugarcane and sugar yield loss that they cause. Many previous studies have shown that the species, their population structure and dominant population of sugarcane borers varied by planting field and growth period, and that could cause the different impacts on sugarcane production, and different loss of cane and sugar yield (White and Hesley, 1987; Milligan et al., 2003; Li et al., 2007; White et al., 2008; Tan et al., 2011; Raza et al., 2012; Goebel et al., 2014). Thus, studying and ascertaining the sugarcane yield and sugar yield loss under natural field conditions when T. schistaceana and C. sacchariphagus occur in mixed populations is important. It can provide detailed data and contribute to effective control of T. schistaceana and C. sacchariphagus.

T. schistaceana and C. sacchariphagus damage in the main sugarcane production area of Yunnan was severe. These results are consistent with previous studies on other borers such as Chilo sacchariphagus and Scirpophaga excerptalis (Rajabalee et al., 1990; Goebel et al., 2014), Diatraea saccharalis (Ogunwolu et al., 1991; White et al., 2008; Rossato et al., 2013), Eoreuma loftini (Legaspi et al., 1999; Reay-Jones et al., 2005) and Eldana saccharina (Goebel and Way, 2003). Previous studies have shown that the mean percent of yield reduction was 14.4%, up to 27.6%, sugar yield loss percent reached 0.7% on average, up as high as 0.8% due to the sugarcane borers in the main production area of Guangxi (Tan et al., 2011); compared with no internodes bored, the sucrose content of internodes bored was reduced by 1.5%–2.9% and gravity purity was reduced by 1.7%–4.3% (Li et al., 2007), and the loss of cane yield caused by sugarcane borer was accounted for 5%~20%, sucrose content was reduced by 0.9% in the main sugarcane production area of Guangdong (Yang, 2003). Thus it can be seen that T. schistaceana and C. sacchariphagus have occurred in combination, leading to considerable damage in the main sugarcane production area of Yunnan recently, the loss of cane and sugar yield caused by T. schistaceana and C. sacchariphagus notably increased, and the main sugarcane cultivars were severely damaged by T. schistaceana and C. sacchariphagus. The damage from T. schistaceana and C. sacchariphagus has become a major challenge that severely impacts on high yield, stable yield and quality of sugarcane. Therefore, the primary task for improving quality, increasing profits, and ensuring the sustainable and stable development of the Chinese sugarcane industry will be the development of an effective control of T. schistaceana and C. sacchariphagus.

Sugarcane plants may be damaged by T. schistaceana and C. sacchariphagus during the whole growing period. Dead heart caused by borers occurred in the seedling stage; the number of seedlings and stalks were reduced which could cause yield reduction. During the middle and later growing stage, the borer damaged stalks and destroyed the internal tissue which had severe impact on the sugarcane quality. In the current study, there was mixed occurrence of T. schistaceana and C. sacchariphagus in the sugarcane planting field in Yunnan that could caused severe damage, and therefore the plant was vulnerable to injury throughout the whole growing season. To control T. schistaceana and C. sacchariphagus effort should be directed towards prevention and integrated control with a focus on both early warning and surveillance. Controlling the first and second generation of T. schistaceana and C. sacchariphagus are likely to be key measures, adopting such practices as, for example, light trapping and biological control to reduce the pest source. At the same time, applying 3.6% Bisultap GR in the seedling phase, the middle and later growing stage should be undertaken.

Acknowledgments

This work was supported by grants from the Earmarked Fund for China Agriculture Research System (CARS-20-2-2), and the Earmarked Fund for Yunnan province Agriculture Research System.

Statement of conflict of interest

The authors declare no conflicts of interest.

References

An, Y.X. and Guan, C.X., 2009. The atlas of sugarcane pests and their control measures. Jinan University Press, Guangzhou.